I’ll echo what Shopping Maul said about ESB taking the series and mythology therein in a deeper and more powerful direction.
Also, both ESB and Raiders of the Lost Ark were written by Lawrence Kasdan at a time when he was absolutely at the peak of his ability. His snappy dialogue and banter in each film is cut from the same cloth as dialogue from some of the absolute greatest films from the 1940’s (Casablanca, The Maltese Falcon, The Thin Man, etc.). It’s an absolute marvel and the actors completely nail their performances.
Also, the technical marvel of things like the Asteroid Chase scene and the Hoth Snow Battle are mind blowing special effects that still hold up when compared to big budget effects movies made 30+ years later!
Basically, ESB is a wonderful fairy tale that adds depth and weight to an already incredible first film while also delivering what is arguably the best story, dialogue, effects, musical score, and character development in the entire series. It is a film made by some of the most talented people to ever work in the medium at a time when they were at the absolute peak of their careers.